1. Field of the Invention
The invention relates to ballistic armor for defeating a projectile. More particularly, the invention relates to a ballistic armor-polyurea composite.
2. Discussion of the Related Art
Polymer-metal composites have been used to produce armor that is lighter in weight than armor made entirely of metal. These composites have been developed to improve blast and shock resistance so that they can be used for ballistic armor to defeat a military projectile.
Polymer composite armor has been developed in combination with metals, ceramics and fiber reinforcements. The polymers were first used in composites to hold dissimilar materials together. It was not initially known that the polymer could contribute physical properties to the composite other than the adhesive property of physically holding dissimilar materials in place to interfere with a projectile. It has been found that polymers can be added to a composite armor to enhance the transfer of shock energy between two non-polymer materials. This is referred to as impedance matching. In contrast, it has been found that polymers can be added to a composite armor to decrease energy transfer. Decreased energy transfer decouples two non-polymer materials from each other. Decoupling is referred to as impedance mismatching.
Momentum trapping armor resulted from the discovery that the order in which a projectile encounters dissimilar materials in a composite armor influences the effectiveness of the armor. A high modulus polymer strike face may reduce the initial velocity of a projectile by a small amount. However that small amount can be enough for the underlying ceramic or metal armor to stop the projectile. In the alternative, a high strength, high elongation polymer can be applied as a backing to function as a spall liner that stops ceramic and metal fragments from becoming impact generated projectiles. In both applications, the polymer is elastically and plastically strained, causing energy adsorption within the polymer.
Polyurea coatings are of interest for coating because they are tougher than urethanes and can be applied to metal surfaces by spray techniques with good adhesion. Higher molecular weight polyureas have been tried for these composite armors. The result has been composites with lower than desirable energy absorption or shock impedance properties.
Generally, high strain rate sensitivity-hardening polyureas useful for composite armor demonstrate a Young's modulus of 1000 psi to 4000 psi when tested at slow strain rates. At high strain rates in the range of 1000/second to 100,000/second, a confined polymer demonstrates a Young's modulus of 350,000 psi to 500,000 psi or greater. When confined, the tensile strength increases from the range of about 2000-8000 psi to about 80,000 psi. Polyureas demonstrating these physical qualities are sold commercially under trade names such as Carboline® POLYCLAD® 707, Air Products VERSALINK® 1000 and SPI POLYSHIELD® Hi-E.
The potential for use of polyurea in composite armor has not been fully realized. Inventor has discovered that problems and deficiencies associated with the use of polyurea in composite armor can be solved or greatly reduced by the selection of polyurea and polyurea mixtures.